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How Mosquitos Get a Whiff of Humans

By transplanting odor-sensing genes from mosquitoes into frog
eggs and fruit flies, scientists have pinpointed dozens of odor
receptors that help mosquitoes sniff out their human prey. The
findings could lead to new methods for repelling or trapping malaria-spreading
mosquitoes through their sense of smell.

Anopheles up close, showing the antennae and feathery mouth
parts that contain odor receptors. Image by Janice Carr,
Centers for Disease Control and Prevention.

Malaria, a disease that kills more than 1 million people each
year, is transmitted by the bite of the Anopheles mosquito.
The insects follow their victims’ scent via odor receptors
found on the mosquito’s antennae and mouth parts. But the
molecular basis of this process has been poorly understood.

To learn more about how Anopheles home in on humans,
collaborators at Yale University and Vanderbilt University conducted
2 separate but complementary studies. They systematically tested
dozens of Anopheles odor receptors for their abilities
to detect 110 odorants. Their research was supported by NIH’s
National Institute on Deafness and Other Communication Disorders
(NIDCD), National Institute of Allergy and Infectious Diseases
(NIAID) and the Foundation for NIH through a grant from the Bill
and Melinda Gates Foundation.

In the study headed by Dr. John R. Carlson at Yale, researchers
transferred 72 Anopheles odor receptor genes, one by one,
into mutant fruit flies that lacked odor receptors in certain nerve
cells. The resulting flies each carried a unique mosquito odor
receptor in these neurons. The flies were exposed to a series of
110 scent molecules, and electrodes recorded the responses of each Anopheles receptor.

As reported in the advance online edition of Nature on
February 3, 2010, the researchers identified several Anopheles receptors
that responded strongly to components of human odor. While most
receptors reacted to many different scents, some seemed specialized
and responded to just one or a few odor molecules.

Most of the tested odors likewise activated many different receptors.
But some biologically important scents—like those that trigger
egg-laying or that are found in human sweat and breath—strongly
activated just a few specific receptors. The study also showed
that some molecules, including 2 found in human sweat, generated
relatively long receptor responses, but others led to briefer activations.

The second study, led Dr. Laurence Zwiebel at Vanderbilt, inserted
the same 72 Anopheles receptor genes into frog eggs, using
a technique previously developed to study the scent receptors of
moths and honeybees. Within a few days, each egg's surface became
dotted with a unique mosquito odor receptor. Receptors that respond
to an odorant cause a reaction within the egg that can be measured.
As reported on February 16, 2010, in the early online edition of
the Proceedings of the National Academy of Sciences, exposing
the eggs to the same panel of 110 odor molecules produced results
similar to those of the fruit fly study.

"We're now screening for compounds that interact with these
receptors," Carlson says. “Compounds that jam these
receptors could impair the ability of mosquitoes to find us. Compounds
that excite some of these receptors could help lure mosquitoes
into traps or repel them. The world desperately needs new ways
of controlling these mosquitoes."